What can I do about my fiancé’s ex-wife’s stipulations on where their children visit?

My fiancé and I are preparing for marriage after our son is born. Our baby’s due date is late November. I have never spoken with his ex-wife for our entire relationship of two years. However, in this last year we have moved in together and the kids stay here at our home when it’s time for him to have them. Their children have expressed to me that they enjoy being here because of the meals we feed them and how we explain to them our reasoning for saying no to something. I never yell at their children or try to physically punish them. Not even so much as a time out. When I have a problem with them, I tell their father and let him handle the children the way he wishes. I do not know what they talk to their mother about. But my fiancé lets me know about the conversations he has with his ex-wife. It wasn't until recently she told my fiancé that she didn't want their children at our home anymore and he would need to find some place else to keep them when it is his time with them. He told her that he wouldn't allow their children to be around someone who would a harm to them or their environment. He also told her it wasn't right that he has to stay someplace that isn't his home just so he can spend time with his children and that there was no place for him to go with the kids to spend the weekend. She didn't care and told him he would need to get a hotel room for the duration of time they would be in his care because he respects her preference when it comes to their children he has struggled to comply, but still gets a room for the weekend. Here's the problem. It takes money from our household that we are already struggling to keep. He is not legally obligated to pay child support but he still gives her at least fifty dollars a week. She makes way more money than he does. Of this new stipulation she has placed on him, we are behind in all or bills. Our home is being threatened by the bank.